Taking a Turn Sometimes, it’s a little harder to see symmetry. Take a sea star. Unlike with a moth or batfish, it’s not so easy to divide a sea star into two matching halves. T e line would have to go right down the middle of one arm. Each half of the sea star would have two whole arms and one half arm. Ouch! Before you split an arm in half, take a


second look at the sea star. T ere’s another way to figure out if it’s symmetrical. See how its five arms stick out from its


center? Each arm is about the same shape and size. All five arms look almost alike. T ey are evenly arranged around the center of the sea star’s body. T is kind of symmetry is called rotational symmetry.


Heads or Tails Here’s an easy way to tell if something has rotational symmetry. Gently pick it up, turn it a little, and put it back down. It should look pretty much like it did before you moved it. T is works with the sea star because it


doesn’t have a head or a tail. Instead, it has a mouth in the middle of its body. Its arms stick out in all directions from that central point. A sea star isn’t the only animal with this


kind of symmetry. A jellyfish has it, too. T is sea creature looks like a shapeless blob when it washes up on a beach. In the water, though, its round body flutters and pulses as it swims. It tosses in the ocean current. Yet no matter how it twists and turns, the round body of a jellyfish always looks the same.


If you pick up this sea star, turn it, and put it down, it still looks the same. That’s a kind of symmetry.


6 NATIONAL GEOGRAPHIC EXTREME EXPLORER


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